It depends only on quantity distribution and number desirability.

For example, if any level has equal quantity of particular item with it, then the more valuable is one that is more likable. For example, level 100 is more likable, because it's the maximum value for the level.

On the halloween items level 13 is somewhat preffered because of it's meaning. And, for example, levels 42 and 69 are just inflated and have no meanings to me what so ever, so as to some other people, so they're less valuable, because less people want them.

The quantity distribution plays lesser role as we see in Max'ses: lvl 1 has far more items than other levels, yet lvl 7 is as invaluable as lvl 1.

So, basically, if you want universal answer, it's "go for 100", and for the other levels research is needed to determine their precise value.

Originally posted by Nedly1:

If Valve made every weapon in the game a possible strange would that ruin the TF2 economy?

It would definitely ruin business model of stocking up huge amounts of weapons that are gonna rise in prices (like ones from crate #40).

And every strange weapon will be automatically one scrap (because of farmers), so there will be no point in opening crates. And if there's no point in opening crates, Valve will lose money. So, it's not gonna happen, until they'll find another way to make keys valuable. Granted that they want to find it, which, I think, they don't.

In my opinion, there's no need to even think about it. As you've described, in the market, these attempts will fail.

The only real way to fix the prices is to get all the items and then sell them at whatever prices you want. But I can't imagine someone buying out any of the discontinued promos. It's too damn expensive.

They've added crates #47 and #48 together. (http://wiki.teamfortress.com/wiki/September_4,_2012_Patch) So they can add salvaged #50 and expiring #51, there's no limitation here.